Typically, an electric brake system is configured to convert the rotation of an electric motor to an axial movement of a friction pad to press the friction pad against a brake disk, thereby generating a braking force. In order to control the braking force to a desired magnitude, many of such electric brake systems include a load sensor mounted at a portion which receives an axially rearward reaction force that acts on the friction pad when the friction pad is pressed against the brake disk.
Such a load sensor mounted in an electric brake system is disclosed in JP Patent Publication 2004-301835A. The load sensor disclosed in that document includes a magnetic target which generates a magnetic field, a magnetic sensor element configured to detect the magnitude of the magnetic field generated by the magnetic target, and a spring configured to be deformed in the axial direction when an axial load is applied. The magnetic target is mounted to the spring, while the magnetic sensor element is fixed in position to face the magnetic target in a direction perpendicular to the axial direction.
The load sensor is configured such that when an axial load is applied to the spring, the magnetic target and the magnetic sensor element are displaced relative to each other in the axial direction. Corresponding to the relative displacement between the magnetic target and the magnetic sensor element, the magnitude of the magnetic field detected by the magnetic sensor element changes. Thus, it is possible to detect the magnitude of the axial load based on the output signal of the magnetic sensor element.
This load sensor has a problem in that since the direction in which the load is applied (axial direction) is the same as the direction in which the magnetic target and the magnetic sensor element are displaced when the load is applied (axial direction), it is difficult to simultaneously achieve both sufficient axial rigidity of the load sensor and sufficient detection accuracy of the load sensor.
That is, in order to increase the axial rigidity of the load sensor, it is necessary to use a spring having high rigidity. However, the higher the rigidity of the spring, the smaller the relative displacement between the magnetic target and the magnetic sensor element, and thus the lower the resolution of the load, i.e. the detection accuracy of the load, tends to be. Conversely, in order to increase the detection accuracy of the load, it is necessary to increase the relative displacement between the magnetic target and the magnetic sensor element by using a spring having lower rigidity, thereby increasing the resolution of the load. This solution, however, results in a reduction in axial rigidity of the load sensor.
Especially if the load sensor is mounted in a vehicle electric brake system, if the load sensor is low in axial rigidity (namely, if the load sensor tends to be deformed markedly in the axial direction when an axial load is applied to the load sensor), this could deteriorate brake response, and/or destabilize feedback control of the load applied to the electric brake. It is therefore important to ensure high axial rigidity of the load sensor. It is also equally important to ensure high detection accuracy of the load.